Process for manufacturing boron nitride agglomerates

ABSTRACT

Disclosed are methods for forming boron nitride-containing aggregates that exhibit improved wear by attrition, and resulting filled polymers that exhibit significantly improved thermal conductivity. The boron nitride-containing aggregates are prepared according to a method that includes wet granulating boron nitride powder with a granulation solution to form wet boron nitride-containing granules; and drying the wet boron nitride-containing granules to cause evaporation of solvent in the granulation solution, thereby forming boron nitride-containing granules. Sintering achieves the desired boron nitride-containing aggregates.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of and claims priorityunder 35 U.S.C. § 120 to U.S. patent application Ser. No. 15/608,509,entitled “PROCESS FOR MANUFACTURING BORON NITRIDE AGGLOMERATES”, byElodie BAHON et al., filed May 30, 2017, which claims priority under 35U.S.C. § 119(e) to U.S. Patent Application No. 62/342,245, entitled“PROCESS FOR MANUFACTURING BORON NITRIDE AGGLOMERATES,” by Elodie BAHONet al., filed May 27, 2016, of which both applications are assigned tothe current assignee hereof and incorporated herein by reference intheir entireties.

FIELD OF THE INVENTION

The present invention relates to boron nitride-containing granules(agglomerates), powders containing sintered boron nitride-containingaggregates, and polymer compositions containing those powders. Methodsof making the boron nitride-containing granules and powders, and polymercompositions containing the same are also disclosed herein.

BACKGROUND OF THE INVENTION

The use of powders of mineral particles as filler in polymers is wellknown in the prior art, this filler making it possible to provideadditional functionalities depending, in particular, on the propertiesof the material constituting the filler. These functionalities are, forexample, increasing the thermal conductivity and/or the hardness and/orthe density of the polymer. The filled polymers thus obtained are used,in particular, in numerous technical fields such as thermal interfacematerials, for example, thermal pastes or thermal dissipaters, orprinted circuit cards.

Due to its high thermal conductivity and electrical resistivity, boronnitride (BN) powders are used in thermal management systems. Among otherpublications, US 2003/0073769, US 2008/0076856, WO 2008/088774 and WO2014/136959 describe such uses. The use of a BN powder is knownparticularly for increasing the thermal conductivity of the polymer,which is particularly desired in thermal interface materialapplications, such as thermal pastes.

To improve the through plane conductivity and reduce the anisotropy ofits properties, BN is used as agglomerates or spherical powder, in therange 20-500 microns. The challenge is to develop a high strength andlow cost agglomerates. Two processes currently used in the industry are:(i) isostatic pressing, crushing, sieving, and firing, which suffersfrom low yield and high costs; and (ii) dispersing, spray-drying, andfiring, which leads to spherical granules presenting very low densityand relatively low strength.

There is a need for a process allowing the manufacturing of BNagglomerates presenting the following properties: yield in the range50-500 micron above 80%, sphericity above 0.8, and improved wearresistance (i.e., low wear by attrition).

The present invention is directed to overcoming these and otherdeficiencies in the art.

SUMMARY OF THE INVENTION

A first aspect of the invention relates to a method for forming boronnitride-containing granules that includes wet granulating boron nitridepowder with a granulation solution to form wet boron nitride-containinggranules; and drying the wet boron nitride-containing granules to causeevaporation of solvent in the granulation solution, thereby formingboron nitride-containing granules.

A second aspect of the invention relates to a boron nitride aggregatepowder that comprises boron nitride-containing aggregates preparedaccording to the first aspect of the invention.

A third aspect of the invention relates to a composition comprising apolymer and a boron nitride aggregate powder according to the secondaspect of the invention.

As demonstrated by the accompanying Examples, the wet granulationprocess of the invention is capable of achieving aggregate BN powder andfilled polymer comprising the powder with improved characteristics. Inparticular, the wear by attrition of the aggregate BN powder is muchlower than the wear by attrition of prior art powders, and the thermalconductivity of the filled polymer is significantly improved.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an image of BN granules prepared in accordance with Example 6,using a granulation solution containing polyethylene glycol 20M andTergitol NP10, and boron nitride powder (PUHP30005, Saint-Gobain). Theimage illustrates the dried BN granules prior to firing.

DETAILED DESCRIPTION OF THE INVENTION

One aspect of the present invention relates to a method for formingboron nitride-containing granules. The method includes wet granulatingboron nitride (BN) powder with a granulation solution to form wet boronnitride-containing granules, and drying the wet boron nitride-containinggranules to cause evaporation of solvent in the granulation solution,thereby forming boron nitride-containing granules. The granules arebasically agglomerates of the particles used during wet granulation.Firing (or sintering) at elevated temperature, as described hereinafter,affords BN- or BN/hybrid aggregates.

The resulting boron nitride-containing aggregates are particularlydesirable, because in comparison to boron nitride-containing aggregatesformed by different processes, including those formed by (i) fluidizedspray drying and (ii) dry granulation with milling, the resulting boronnitride-containing aggregates have high sphericity, highdensity/strength, low porosity, or a combination of two or more of theseattributes. In addition, the resulting boron nitride-containingaggregates have a high yield of aggregates within the desired size-rangeof less than about 500 microns, particularly between about 50 and about500 microns, or between about 50 and about 400 microns. Yields in excessof 50% can be obtained using the process as disclosed herein. Inpreferred embodiments, yields in excess of 60%, 65%, 70%, 75%, or 80%can be obtained using the process as disclosed herein. Improvedproperties for both unfired and fired boron nitride-containinggranules/aggregates can be achieved.

Wherever the word “about” is employed herein in the context ofdimensions (e.g. particle sizes, and weights), amounts (e.g. relativeamounts of individual constituents in a composition or a component of acomposition, including ratios of such constituents or components),temperatures, pressures, times, or concentrations, it will beappreciated that such variables are approximate and as such may vary by±10%, for example ±5%, ±4%, ±3%, ±2%, or ±1% from the numbers specifiedherein.

“Aggregate” is understood to mean a group of particles including BN,said particles being assembled together and strongly linked rigidly, inparticular by sintering, so as to constitute the individualized grainsreferred to as aggregates, which constitute said powder.

On the other hand, “agglomerate” is understood to refer to an assemblyof particles that are weakly linked and readily dispersible.

“Consisting essentially” is understood to mean that the very largemajority of said powder consists of said boron nitride-based aggregatesor mixed-composition aggregates, but without excluding the presence ofparticles that are not boron nitride-based aggregates, such as boronnitride elementary particles, which are however necessarily present invery small quantity. More particularly, it is understood that the powdercontains more than 90% by weight, preferably more than 95% by weight, oreven more than 99% by weight of said boron nitride-based aggregates.Naturally, according to a possible embodiment, the powder consists onlyof boron nitride-based aggregates, except for the inevitable impurities.

The process of the present invention relies on the use of startingmaterials including BN powder; optionally an additional powder materialsuch as a metal oxide powder, or precursor of metal oxide (in powderform), in which case the resulting BN-containing granules are hybridgranules (i.e., containing both BN and the metal oxide); and agranulation solution that is used during the wet granulation step. Thesematerials are separately discussed below.

Boron nitride (BN), a manufactured ceramic, has excellent heatconductivity, chemical stability, electrical insulation, inertness, andmachinability. Boron nitride powders are synthesized using a number ofboron sources, including but not limited to boron oxide, borax, boricacid, and calcium hexaboride. BN powders are available with varyingpurity levels. The powder may contain impurities such as boron oxide,carbon, oxygen, metals, and other impurities. BN powders arecommercially available from a number of sources, including Saint-Gobain(Amherst, N.Y.).

Mechanical, thermal and dielectric properties of the resultingBN-containing granules can be influenced by levels of initialimpurities, such as oxygen. For certain applications, the presence ofoxygen is detrimental to the quality of the BN-containing granules, andfor other applications the presence of oxygen may be beneficial.

In one embodiment, the initial BN powder includes powder grains of BNhaving an oxygen content of less than or equal to 10% by weight, acalcium content of less than about 1000 ppm by weight, or both.

In another embodiment, the initial BN powder includes powder grains ofBN having an oxygen content of less than or equal to 5% by weight, acalcium content of less than about 600 ppm by weight, or both.

The composition of the initial BN powder may be adjusted according tothe desired resulting granule properties, including but not limited to:density, sphericity, size and final composition.

In certain embodiments, only BN powder is introduced during the wetgranulation step. In these embodiments, the BN powder is essentiallyfree of metallic or semi-conductor material and, hence, the resultingwet boron nitride-containing granules are free or essentially free ofmetallic or semi-conductor materials. See U.S. Patent Application Publ.No. 20090304943 for a description of thermal spraying using a freeflowing agglomerate containing a ceramic component that sublimes (suchas boron nitride), a metallic or semi-conductor material that does notsublime, and a binder.

Where hybrid BN-containing granules are desired, the optional metaloxide powder, or precursor of metal oxide, is also introduced before orduring the wet granulation step. Exemplary metal oxide powders include,without limitation, SiO₂, Al₂O₃, TiO₂, and rare earth oxide powders, aswell as combinations of any two or more thereof. Powdered forms of SiO₂,Al₂O₃, TiO₂, and rare earth oxides are commercially available from anumber of sources.

In one embodiment of the present invention, the weight ratio of BNpowder to metal oxide powder is between about 0.1:1 to about 10:1. Inother embodiments the weight ratio of BN powder to metal oxide powdermay be between about 1:1 to about 10:1.

When hybrid BN granules are desired, the BN powder and metal oxidepowder can optionally be dry mixed prior to the wet granulation step.Dry mixing can be carried out for any suitable period of time, e.g.,about 5 minutes up to about 60 minutes, or longer if desired.

In alternative embodiments, no dry mixing is carried out when using twoor more powder materials during the wet granulation step. In thisembodiment, any mixing of the two or more powders occurs during the wetgranulation step.

The granulation solution is any liquid that is suitable to wet theinitial BN powder (or mixture of powders). The granulation solutionincludes a solvent and may optionally contain additional components,including but not limited to surfactants and binders.

The granulation solution can be formed prior to the step of wetgranulating by combining the solvent and the additional components indesired amounts. Alternatively, the granulation solution can be formedin situ during the step of wet granulating by combining with the powdermaterials, in any desired order, the solvent and optional binders andsurfactants. Thus, binder and surfactant can be added to the powderbefore or during the wet granulation step, either separately from thesolvent or with the solvent.

The solvent comprises any materials that can be used in liquid or vaporforms, which can partially or wholly dissolve any additional componentsof the granulation solution, and which can be wholly or partiallyremoved by drying, firing, or a combination of both drying and firing.Exemplary solvents include, without limitation, water, organic solvents,and meltable binders.

In one embodiment, the granulation solution comprises water as thesolvent, a surfactant, and a binder. In other embodiments, thegranulation solution can optionally comprise an organic solvent that issufficiently soluble in water.

Surfactants are compounds that lower the surface tension between twoliquids or between a liquid and a solid. Surfactants may act asdetergents, wetting agents, emulsifiers, foaming gents, and dispersants.In wet granulating, a surfactant may be used in the granulation solutionor, as noted above, introduced separately during the step of wetgranulation.

In one embodiment of the present invention, the surfactant is anon-ionic surfactant. Exemplary classes of non-ionic surfactantsinclude, without limitation, alkoxylated (alkyl)phenols, polycarboxylicacids, silanes, and organometallic compounds. Commercially availablemembers of these exemplary classes are nonylphenol ethoxylate,polycarboxylic acids (e.g., Rhodaline 111M™ available from Rhodia, Inc.,Cranbury, N.J.), silanes (e.g., Z-6040 Silane™ available from DowChemical, Midland Mich.), and organometallic compounds (e.g., APG™available from Cavedon Chemical Co., Woonsocket, R.I.).

In one embodiment of the present invention, the granulation solutioncontains between about 0.1 to about 5.0 percent (by weight) surfactant,between about 0.2 to about 4 percent (by weight) surfactant, or betweenabout 0.3 to about 2 percent (by weight) surfactant.

Binders are any material used to facilitate the agglomeration of wetmass by adhesion. In wet granulating, a binder may be used in thegranulation solution or, as noted above, introduced separately duringthe step of wet granulation.

Exemplary binders include, without limitation, polyethylene glycol,polyvinylalcohol, latex, silicone oil, and epoxy resin.

In one embodiment of the present invention, the granulation solutioncontains between about 1.0 to about 10.0 percent (by weight) binder,between about 1.0 to about 5.0 percent (by weight) binder, or betweenabout 1.0 to about 4.0 percent (by weight) binder.

Wet granulation is a process by which small particles are converted intoagglomerates, and is achieved by bringing the initial powder particlesin intimate contact with a granulation solution with or without abinder. This step can be carried out using, e.g., high or low shearmixers, high or low speed mixers, fluidized bed granulators, tumblingdrums or extruders, or a combination thereof.

Wet granulation processes include but are not limited to: reverse wetgranulation, steam granulation, moisture-activated dry granulation,thermal adhesion granulation, and melt granulation (see Shanmugam,“Granulation techniques and technologies: recent progress,” BioImpacts,5(1):55-63 (2015), which is hereby incorporated by reference in itsentirety) as well as extrusion and high/low shear granulation.

In one embodiment of the present invention, wet granulating is carriedout using a high sheer mixer. High shear mixers disperse or transportone phase or ingredient into a main continuous phase, with which itmight normally be immiscible, and vary in design and configuration.Generally, a moving rotor and/or impeller, a group of rotors and/orimpellers, or a series of rotors and/or impellers, together with astationary component is used to create shear. High shear mixers may bedesigned as in-line units, batch units, or a combination thereof, andmay use various configurations, including but not limited to:rotor-stator teethed, blade-screen, radial-discharged oraxial-discharged.

Wet granulating carried out using high shear mixers may be carried outusing any of a variety of mixing speeds, whether the mixing speed isvaried over the duration of the wet granulation step or maintainedconstant for the duration of the wet granulation step. In one embodimentof the present invention, wet granulating is carried out using animpeller radial speed of at least about 15 meters per second (m/s), orbetween about 20 m/s up to about 30 m/s.

The wet granulation step can be carried out for any suitable length oftime, which is sufficient to result in agglomerates or granules fallingwithin a particular size profile. In one embodiment of the presentinvention, wet granulating is carried out for up to about 60 minutes. Inother embodiments, wet granulating is carried out for up to about 30minutes, up to about 20 minutes, up to about 15 minutes, or up to about10 minutes. Wet granulating steps carried out for anywhere from about 1to about 10 minutes is also contemplated.

The ratio of the granulation solution to powder fraction may vary.Desirably, under most circumstances, the amount of granulation solutionintroduced during the wet granulation step should not form a slurry.

In one embodiment of the present invention, wet granulating is carriedout with a liquid:solids ratio (by volume) of between about 0.1:1 toabout 1:1, about 0.2:1 to about 1:1, about 0.2:1 to about 0.6:1, orabout 0.3:1 to about 0.55:1. In certain embodiments, a liquid:solidsratio (by volume) of between about 0.1:1 to about 0.5:1 can be used. Asused herein, the liquid:solids ratio refers to the total amount ofliquid by volume to the total amount of powder by volume used during thewet granulation step. As discussed below, the granulation solution maybe added over the course of time during the wet granulation step, inwhich case the ratio at the beginning would differ from the ratio thatexists at the end of the wet granulation step. As such, reference to thetotal or final liquid:solids ratio is contemplated.

Where the powder materials introduced during the wet granulation stepinclude both BN powders and metal oxide powder, or precursor of metaloxide, liquid:solids ratios falling within the ranges of the precedingparagraph can be employed.

As an alternative measure of the liquid:solids ratio, a liquid to powderpore volume ratio can alternatively be used. In certain embodiments, wetgranulating is carried out with a liquid:powder pore volume of about0.2:1 to about 2:1, about 0.4:1 to about 1.5:1, or about 0.5:1 to about1:1.

Addition of the granulation solution to the powder fraction, i.e., BNpowder or powder mixtures, may happen immediately prior to wetgranulating, periodically during wet granulating, or at any rate duringwet granulating. In one embodiment of the present invention, thegranulation solution is introduced continuously to the BN powder (orpowder mixture) during the wet granulating step. For example, thegranulation solution can be introduced continuously for a first periodof time followed by continued wet granulation for a second period oftime without the introduction of additional granulation solution.Alternatively, the granulation solution can be introduced continuouslyand then the wet granulation processed halted immediately (i.e., within1 minute) following completion of the granulation solution introduction.

Following the wet granulating step and prior to drying, the sphericityand density of granules may be improved by the addition of an optionalpowder dusting step. Powder dusting involves mixing the granulated wetagglomerates at reduced speed while introducing additional BN powder,metal oxide powder (or precursor thereof), or a mixture of BN and metaloxide (precursor) powders. By way of example, reduced speed of themixing process contemplates an impeller speed of not more than about 10m/s, or between about 1 and about 10 m/s.

One example of the powder dusting step contemplates low speed mixing ofthe wet boron nitride-containing granules with dry boron nitride powder,dry metal oxide powder, or a mixture of such powders.

Another example of the powder dusting step contemplates low speed mixingof the wet boron nitride/metal oxide-containing granules with dry boronnitride powder, dry metal oxide powder, or a mixture of such powders.

Following wet granulating (with or without the powder dusting step), themixture is dried. Drying can take place under any condition that allowsthe solvent of the granulation solution to be wholly or partiallyremoved. Preferably, the dried product is essentially free of thesolvent used to form the granulation solution. By “essentially free”, itis contemplated that the solvent content of the dried granules is lessthan about 2 percent, less than about 1.5 percent, less than 1 percent,less than about 0.5 percent, or less than about 0.1 percent.

Drying conditions, including but not limited to temperature, timeduration, and atmospheric conditions, such as pressure and medium, maybe constant or may be adjusted throughout the process. For example,drying may be carried out for several hours to overnight. Drying can beachieved by exposing the agglomerate material to infrared heating,microwave heating, reduced pressure (vacuum) environment, or acombination thereof.

In one embodiment of the present invention, the drying process includesdrying at a first temperature for a first period of time and firing (orsintering) the initially dried product at a second temperature for asecond period of time. In one embodiment, drying at the firsttemperature is between about 20° C. and about 200° C., and it is carriedout overnight. In one embodiment, firing is carried out in inert orpartially reducing atmosphere at a temperature of between about 1600° C.and about 2200° C., and it is carried out for at least one hour.Thereafter, the resulting fired product, the BN-containing aggregates orhybrid BN/metal oxide aggregates, can be cooled gradually to roomtemperature.

In an alternative embodiment, the drying and firing steps can be carriedout successively during the ramp up phase and subsequent firing (orsintering) phase of the firing process.

Depending on the process parameters employed during the wet granulationsteps, certain properties of the resulting BN-containing aggregates canbe controlled, including sphericity, density/strength, porosity, sizevariation, and yield. If desired, however, the resulting sinteredaggregates can optionally be subjected to grinding to obtain particlesizes within a particularly preferred size range, with particle sizeselection as is well known in the art.

Further, as noted above, the level of impurities in the starting BNpowder can affect the properties of the final (fired) BN aggregates. Inone embodiment, the fired BN aggregates have an oxygen content of lessthan or equal to 2% by weight, a calcium content of less than 400 ppm byweight, or both. In another embodiment, the fired BN aggregates have anoxygen content of less than 1.5% by weight, a calcium content of lessthan 300 ppm by weight, or both.

In one embodiment, the resulting powder (or mixture) consistsessentially of boron nitride-based aggregates, said powder having:

a) the following overall chemical composition, in percentages by weight:

between 40 and 45% inclusive of boron,

between 53 and 57% inclusive of nitrogen,

less than 5%, in total, of other elements,

a calcium content of less than 400 ppm by weight;

b) a structural composition including more than 90% inclusive of boronnitride, in percentages by weight and on the basis of the totality ofthe crystallized phases present in said powder, and

c) the following physical properties:

a mean circularity of the aggregates greater than or equal to 0.90,

a median pore size less than or equal to 0.3 μm,

an open porosity less than or equal to 55%.

In the present invention, the elements other than 0, C and N of saidchemical composition, in particular, boron and calcium, are measuredconventionally using the aggregate powder by inductively coupled plasmaatomic emission spectrometry (ICP-AES).

In the present invention, the elements O, C and N of said chemicalcomposition are measured conventionally using the aggregate powder byinfrared spectrometry for the elements O and C and by thermalconductivity for the element N, for example, using a LECO series TC436DR apparatus for the elements N and O and a LECO series SC 144DRapparatus for the element C.

In the present invention, said structural composition is obtainedconventionally from the aggregate powder by X-ray diffraction andRietveld refinement.

According to one embodiment, the powder is characterized by:

the content by weight of boron is greater than or equal to 41%;

the content by weight of boron is less than or equal to 44%;

the content by weight of nitrogen is greater than or equal to 54%;

the content by weight of nitrogen is less than or equal to 56%;

the calcium content is less than 330 ppm, or less than 300 ppm byweight, preferably less than 200 ppm by weight, preferably less than 100ppm by weight, or even more preferably less than 50 ppm by weight.

In a preferred embodiment, the content by weight of elements other thanthose described in said elementary chemical formulation given above isless than 4%, preferably less than 2%, preferably less than 1%,preferably less than 0.5%, preferably less than 0.1%. In saidembodiment, these elements are preferably impurities, that is to sayelements that are added unintentionally, for example, by the rawmaterials used in the feedstock, such as the elements O, C, Mg, Fe, Si,Na, K.

Preferably, the oxygen content in the powder is less than 5000 ppm byweight, preferably less than 2000 ppm by weight, or even less than 1000ppm by weight.

In a particular embodiment, said other elements include a sinteringadditive for the boron nitride in a quantity preferably greater than orequal to 0.5%, preferably greater than 1% and less than 4%, preferablyless than 3%, preferably less than 2%.

The sintering additive for the boron nitride is selected from LaB₆; theoxides of rare earths, of elements of columns 3 and 4 of the periodictable of elements and their mixtures; the nitrides of the elements ofcolumn 4 of the periodic table of elements; and their mixtures.Preferably, said sintering additive is selected from LaB₆, Y₂O₃, thenitrides of the elements Ti, Zr, Si, Al and their mixtures. Preferablysaid sintering additive is selected from LaB₆, Y₂O₃, the nitrides of theelements Ti, Si, Al, and their mixtures.

The powder according to the invention contains quite preferably a boronoxide B₂O₃ content less than 5%, preferably 2%, more preferably lessthan 1%, or even less than 0.5%, and quite preferably less than 0.1%.

The boron oxide B₂O₃ content of the powder according to the invention ismeasured conventionally by titration with mannitol.

In the present invention, the following are conventional meanings:

“sintering additive” for boron nitride is understood to mean a compoundfacilitating the sintering of said boron nitride, for example, byreducing the temperature needed for said sintering, by improving thedensification or by limiting crystal growth;

“rare earth” is understood to mean an element of the group of thelanthanides plus scandium Sc and yttrium Y; and

“lanthanide” is understood to mean an element having an atomic numberbetween 57 (lanthanum) and 71 (lutetium) of the periodic table.

According to preferred embodiments of the present invention:

said structural composition includes more than 95%, preferably more than98%, of boron nitride, in percentages by weight and on the basis of thetotality of the crystallized phases present in said powder;

said structural composition includes more than 90%, preferably more than95%, preferably more than 98%, of boron nitride, in percentages byweight and on the basis of the weight of said powder; or

the boron nitride is present at more than 60%, preferably more than 70%,preferably more than 80%, or even approximately 100% in the form of ahexagonal structure, in percentages by weight and on the basis of thecrystallized boron nitride phases present in said powder.

In said physical properties:

the aggregate powder has a mean circularity greater than or equal to0.92, preferably greater than or equal to 0.93, or even greater than orequal to 0.94, or even greater than or equal to 0.95;

the aggregate powder has a median pore size less than or equal to 0.25μm, preferably less than or equal to 0.2 μm and preferably greater than0.05 μm; and/or

the aggregate powder has an open porosity less than 53%, preferably lessthan 50%, preferably less than 49%, or even less than 47%, or even lessthan 45%.

For the evaluation of the circularity “Ci” of an aggregate P, onedetermines the perimeter P_(D) of the disk D having an area equal to thesurface area A_(p) of the aggregate P in a picture of this aggregate. Inaddition, the perimeter P_(r) of this aggregate is determined. Thecircularity is equal to the ratio P_(D)/P_(r) or

${Ci} = {\frac{2*\sqrt{\pi\; A_{p}}}{\Pr}.}$The more elongate the shape of the aggregate is, the lower the firstcircularity is.

The mean circularity of an aggregate powder in the sense of the presentinvention corresponds to the arithmetic mean of the different valuesobtained for the population of aggregates constituting the powder.

All the known measurement methods for evaluating the circularity can beconsidered and, in particular, a manual or automated observation ofphotographs of the aggregate, for example, using a Morphologi® G3Sapparatus marketed by the company Malvern. Such an apparatus also makesit possible to determine the mean circularity of an aggregate powder.

The median size of the pores of the aggregate powder is evaluated bymercury porosimetry according to the standard ISO 15901-1. The term“median size” of a set of pores, denoted D₅₀, is understood to mean thesize dividing the pores of this set into a first and a second populationof equal volume, these first and second populations comprising onlypores having a size greater than or less than said median size,respectively.

The porosity of the aggregate powder is evaluated conventionally bymercury porosity according to the standard ISO 15901-1.

According to other preferred embodiments of the present invention:

the aggregate powder has a median size greater than 30 μm, preferablygreater than 50 μm and less than 500 μm, preferably less than 400 μm,preferably less than 300 μm, preferably less than 200 μm; in anembodiment, the median size is between 40 μm and 70 μm, and in anotherembodiment, the median size is between 100 μm and 150 μm;

the aggregate powder has a maximum size less than 1 mm, preferably lessthan 750 μm;

the aggregate powder has a percentile D₁₀ greater than 5 μm, preferablygreater than 10 μm, preferably greater than 20 μm; and/or

the aggregate powder has a ratio (D₉₀−D₁₀)/D₅₀ less than 10, preferablyless than 5, or even less than 3, or even less than 2; advantageously,the pourability of the powder is improved as a result.

Preferably, the aggregates comprise randomly oriented boron nitrideplatelets. The properties of said aggregates are then essentiallyisotropic.

“Median size” of a set of aggregates (or of grains), denoted D₅₀, isunderstood to mean the size dividing the aggregates (the grains) of thisset into a first and a second population of equal weight, these firstand second populations comprising only aggregates (the grains) having asize greater than or less than said median size, respectively.

“Percentiles” 10 (denoted D₁₀), 90 (denoted D₉₀) and 99.5 (denotedD_(99.5)) is understood to mean the sizes of aggregates (the grains),corresponding to the percentages of 10%, 90% and 99.5% by weight,respectively, on the cumulative particle size distribution curve of thesizes of aggregates (the grains) of the powder, said sizes of aggregates(the grains) being classified in increasing order. According to thisdefinition, 10% by weight of the aggregates of the powder thus have asize less than D₁₀ and 90% of the aggregates, by weight, have a sizegreater than D₁₀. The percentiles are determined using a particle sizedistribution obtained using a laser granulometer.

“Maximum size” of a powder is understood to mean the 99.5 percentile.

The particle size distribution of the powders of aggregates (of grains)according to the invention is, for example, determined by laserdiffusion using a Camsizer granulometer marketed by the company Retschtechnologies, without suspending said powder beforehand. From thisparticle size distribution, the following are conventionally determined:the median size D₅₀, the percentile 10 (D₁₀), and the percentile 90(D₉₀), as well as the maximum size (D_(99.5)).

The resulting BN-containing aggregates (including hybrid aggregates)find use as thermally conductive, electrically insulated fillers inthermal management applications, as thermal sprayable powders, or asfeed for manufacturing BN solid blocks. In one embodiment, the aggregatepowder according to the invention as described above is used as fillerdispersed in a polymer matrix, thereby forming a compositeaggregate-polymer material.

Microelectronic devices, such as integrated circuit chips, are becomingsmaller and more powerful. The current trend is to produce integratedchips which are steadily increasing in density and perform many morefunctions in a given period of time over predecessor chips. This resultsin an increase in the electrical current used by these integratedcircuit chips. As a result, these integrated circuit chips generate moreohmic heat than the predecessor chips. Accordingly, heat management hasbecome a primary concern in the development of electronic devices.

Typically, heat generating sources or devices, such as integratedcircuit chips, are mated with heat sinks to remove heat which isgenerated during their operation. However, thermal contact resistancebetween the source or device and the heat sink limits the effective heatremoving capability of the heat sink. During assembly, it is common toapply a layer of thermally conductive grease, typically a siliconegrease, or a layer of a thermally conductive organic wax to aid increating a low thermal resistance path between the opposed matingsurfaces of the heat source and the heat sink. Other thermallyconductive materials are based upon the use of a binder, preferably aresin binder, such as a silicone, a thermoplastic rubber, a urethane, anacrylic, or an epoxy, into which one or more thermally conductivefillers are distributed.

Typically, these fillers are one of two major types: thermallyconductive, electrically insulative or thermally conductive,electrically conductive fillers. Aluminum oxide, magnesium oxide, zincoxide, aluminum nitride, and boron nitride are the most often citedtypes of thermally conductive, electrically insulative fillers used inthermal products. BN-containing granules or aggregates (including hybridgranules or aggregates) are especially useful because they haveexcellent heat transfer characteristics and are relatively inexpensive.

Preferably, in the composite according to the invention, the content byweight of aggregates is greater than 20%, preferably greater than 30%and preferably less than 80%, preferably less than 70%, on the basis ofthe weight of the filled polymer.

In such a filled polymer, the polymer can be selected, in particular,from the thermosetting polymers, the thermoplastic polymers. Preferably,the polymer is selected from the thermosetting polymers. Also,preferably, the thermosetting polymer is selected from the epoxy resinsand the silicones. The thermoplastic polymer is preferably selected frompolytetrafluoroethylene or PTFE, phenylene polysulfide or PPS, polyetherether ketone or PEEK, butylene polyterephthalate or PBT, the nylons, thepolycarbonates and the elastomers.

Without going beyond the scope of the invention, the powder containingboron nitride-based aggregates according to the invention can be mixedbeforehand, before its introduction into said polymer, with anotherpowder, for example, a powder of alumina aggregates. In other words, thepresent invention also relates to any powder mixture including thepowder consisting essentially of above-described boron nitride-basedaggregates.

Examples 1-4

The invention and its advantages will be better understood upon readingthe embodiment examples below, which are provided only for illustrativepurposes and which do not limit the present invention.

The boron nitride aggregate powder according to Comparative Example 1 isa PCTL5MHF powder marketed by Saint-Gobain Boron Nitride.

The boron nitride aggregate powder according to Comparative Example 2 isa PCTH7MHF powder marketed by Saint-Gobain Boron Nitride.

The boron nitride aggregate powder according to Comparative Example 3 isproduced by using the following process: a boron nitride powder, havingan oxygen content of 5% by weight, a calcium content of 100 ppm, acontent of elements other than oxygen and calcium less than 1% byweight, is crushed under dry conditions in a ball mill so that it has amedian size of 3 μm. The powder is pressed in the form of pellets havinga diameter of 50 mm using an isostatic press at a pressure of 200 MPa.The relative density of the pellets obtained is equal to 45%. Thepellets obtained are then crushed by means of a roller mill then sievedto 90 μm and to 45 μm, and finally subjected to a heat treatment undernitrogen in a cycle having a rise rate of 100° C./h at 2000° C., aholding time of 2 h at this temperature, and a descent at 300° C./h. Inthe end, the powder thus obtained is sieved so as to keep the particlesize range between 45 μm and 90 μm.

The boron nitride aggregate powder according to Example 4, according tothe present invention, is produced by the following process: aSaint-Gobain Boron Nitride PUHP30005 boron nitride powder having anoxygen content of 1% by weight, a calcium content of 100 ppm, a mediansize of 1 μm, is pressed in the form of pellets having a diameter of 50mm using an isostatic press at a pressure of 200 MPa. The relativedensity of the pellets obtained is equal to 75%. The pellets obtainedare then crushed by means of a roller mill, then ground in a ball millin which the balls have been removed, for 1 hour, said mill rotating ata speed of 5 rpm, then sieved, with application of ultrasound, to 90 μmand to 45 μm, and finally subjected to a heat treatment under nitrogenin a cycle having a rise rate of 100° C./h at 2000° C., a holding timeof 2 h at this temperature, and a descent at 300° C./h. In the end, thepowder thus obtained is sieved so as to keep a particle size rangebetween 45 μm and 90 μm.

Table 1 below lists the properties of the powders of Examples 1 to 4,after elemental, structural and physical analyses carried out by meansof the techniques described above.

TABLE 1 1(*) 2(*) 3(*) 4 Chemical analysis (weight data) B (%) 43 43 4343 N (%) 56 56 56 56 Ca (ppm) 500 300 20 20 other elements (%) <1 <1 <1<1 Among which O (ppm) 1000 1000 1000 1000 Among which C (ppm) 100 100100 100 Crystallized phases present and quantity in % on the basis ofthe crystallized phases boron nitride 100 100 100 100 Other physicalproperties Circularity 0.77 0.75 0.9 0.93 Open porosity (%) 57 50 58 48Median size of the pores (μm) 0.75 0.7 0.16 0.18 D₅₀ of the particles(μm) 80 100 110 90 (*)not according to the invention

The boron oxide content as measured by titration with mannitol is on theorder of 0.1% for all the examples.

The wear by attrition of the powders obtained according to Examples 1 to4 is then estimated using the following test: 20 g of aggregate powderpassing through the mesh openings of a sieve with 90 μm openings and notpassing through the mesh openings of a sieve with 45 μm openings areplaced in a closed nylon container, so that said powder occupies 45% ofthe volume of said container. The container is then stirred for 120minutes at a rotational speed of 20 rpm in a jar turner. After the test,the weight of the particles passing through the mesh openings of a sievewith 45 μm openings is determined. It corresponds to the quantity offine particles created in the test. This quantity of fine particlesgenerated, or “wear by attrition,” is expressed as percentage of theweight of the powder before the test. The higher said quantity of fineparticles generated during the test is, the greater the wear byattrition of the aggregate powder is.

A wear by attrition greater than 20% is believed to lead to anappreciable decrease of the thermal conductivity of the filled polymercomprising said aggregates. Preferably, the wear by attrition is lessthan 15%, preferably less than 10%.

The decrease of the wear by attrition of an aggregate powder A incomparison to an aggregate powder B is equal to the difference betweenthe wear by attrition of powder A and the wear by attrition of powder B,divided by the wear by attrition of powder A, expressed as percentages,powder A being the powder considered as reference.

The powders obtained according to Examples 1 to 4 are then used asfiller in an ordinary polymer matrix of the TSE3033 silicone resin typemarketed by Momentive Performance Materials. The inclusion and thedispersion of the aggregates in the polymer matrix are carried outaccording to the following protocol:

Each powder is dispersed in the TSE3033 silicone resin (the two parts Aand B of the resin being mixed in equal quantity, by weight) at ambienttemperature in a Rayneri VMI Turbotest mixer marketed by the companyVMI, at a speed of rotation of 200 revolutions per minute. The weight ofpowder introduced is equal to 40%, on the basis of the sum of the weightof the TSE3033 silicone resin and the weight of the powder. Each mixturethus obtained is then cast so as to obtain a film having a thickness of5 mm. Said film is then heated at a temperature of 100° C. for a timeperiod of 2 hours.

Measurements of the through plane thermal conductivity are carried outon each polymer film obtained, the “through plane,” in English, thermalconductivity designating the thermal conductivity measured along thedirection perpendicular to the polymer film, in other words measuredalong the thickness of said film.

The measurements are carried out according to the following standardsand experimental protocols:

The thermal conductivity is given conventionally as the product of thediffusivity, the density and the thermal capacity.

More particularly, according to the invention, the “through plane”thermal conductivity is measured as the product of the through planethermal diffusivity, of the density and of the thermal capacity.

The thermal diffusivity of the polymers is measured according to thestandard ASTM C-518 using the thermal flow method. The diffusivity ismeasured perpendicularly to the polymer layer (through plane thermaldiffusivity).

The thermal capacity of the polymers is measured by differentialscanning calorimetry, in English, (DSC) using a Netzsch thermobalance.

The density of the polymers is measured by helium pycnometry.

The improvement of the thermal conductivity of a filled polymercomprising an aggregate powder A compared to a filled polymer comprisingan aggregate powder B is equal to the difference between the thermalconductivity of the filled polymer comprising powder B and the thermalconductivity of the filled polymer comprising powder A, divided by thethermal conductivity of the filled polymer comprising powder A,expressed as percentages (the filled polymer comprising powder A beingthe reference filled polymer).

The results of the wear by attrition tests of the aggregate powders andof the thermal conductivity measurements are given in the followingTable 2:

TABLE 2 Thermal Decrease Conductivity Improvement Open Mean Pore CalciumWear by of Wear by of Filled of Thermal Porosity Size Content AttritionAttrition† Polymer Conductivity‡ Example Circularity (%) (μm) (ppm) (%)(%) (W/m · K) (%) 1(*) 0.77 57 0.75 500 31 — 0.5 — 2(*) 0.75 50 0.7 30023 26 0.8 60 3(*) 0.9 58 0.16 20 25 19 0.5 0 4 0.93 48 0.18 20 7 77 1100 †Compared to Powder of Example 1 ‡Compared to Polymer Filled withPowder of Example 1 (*)Not according to the invention

The data given in Table 2 show that the powder of boron nitride basedaggregates according to the invention has a measured wear by attritionof 7% and that the filled polymer obtained from the powder of boronnitride based aggregates according to the invention has a through planethermal conductivity that is much higher than that of all the othersamples.

More specifically, the aggregate powder according to Example 1 inaccordance with the teaching of the document WO2014/136959 does notreach the desired compromise. In particular, the wear by attritionappears higher than that of Example 4 of the present invention.

The best compromise is also not reached by the aggregate powderaccording to Example 2 (not according to the invention) and for thefilled polymer comprising said powder. Although lower than that ofExample 1, the wear by attrition of the aggregate powder is still toohigh. However, the thermal conductivity of the filled polymer comprisingsaid powder is substantially higher (by 60%) than that of the filledpolymer comprising the powder according to Example 1.

As for the aggregate powder according to Example 3 (not according to theinvention), which includes no calcium, and the filled polymer comprisingsaid powder, one notes that the wear by attrition of the aggregatepowder is still too high and that the thermal conductivity of the filledpolymer comprising said powder is not improved in comparison to that ofthe filled polymer comprising the powder according to Example 1.

The best results and compromise are obtained for the aggregate powderaccording to Example 4 according to the present invention and the filledpolymer comprising said powder. The wear by attrition of the aggregatepowder is limited to only 7%, that is to say much lower than the wear byattrition of the aggregate powders of Comparative Examples 1, 2 and 3.The thermal conductivity of the filled polymer comprising the aggregatepowder according to Example 4 is, in particular, on the order of twicethat of the filled polymer comprising the powder according to Example 1and, in absolute value, it is the highest of all the samples tested.

A comparison between the aggregate powders of Examples 1 and 4 thusmakes it possible to demonstrate the significant improvement obtainedaccording to the invention by a specific adjustment of the parameters ofopen porosity, calcium content, circularity of the aggregates, and meanpore size.

In particular, in regard to the teaching of the above-described priorpublication WO2014/136959, the applicant company has demonstrated thecumulative impact of the decrease of the mean pore size, the decrease ofthe calcium content, the decrease of the open porosity, and the increaseof the circularity on the properties of resistance to attrition andthermal conductivity of the filled polymer comprising said powders.

In particular, if one compares the Examples 1 (according toWO2014/136959) and 4 (according to the invention), one notes that:

the wear by attrition decreases from 31% to 7%, which corresponds to a77% decrease; and

the thermal conductivity of the filled polymer increases from 0.5 to 1W/m·K and is improved by 100%.

Unexpectedly, in view of the better compromise obtained by means oftheir adjustment according to the present invention, the inventors thushave demonstrated a true synergy between the properties of circularity,open porosity, mean pore size and calcium content.

Examples 5-8

Granulation was carried out in high shear mixers in a batch processconsisting of a bowl equipped with a multi blade impeller and a scraper.The feed powder, Saint-Gobain Boron Nitride PUHP30005 boron nitridepowder, was dry mixed before the process to ensure homogenous startingconditions of each batch.

Binder was then added either at the beginning or gradually during theprocess. Binder can be added in a drop by drop fashion or sprayed on. Anadditional step of powder dusting can be added to improve the sphericityand the density of the granules. Finally, the wet mixing stage isfollowed by a drying stage.

Particle Size Distribution and Spherical Coefficient (Camsizer):

Camsizer XT (Horiba, Japan) was used to obtain the particles sizedistribution (d10, d50 and d90) of the granules and the shape(sphericity coefficient).

Morphology (SEM):

The granule morphology was observed on a SEM TM-1000 (HitachiHigh-Technologies Co, Japan).

Porosity:

Porosimeter Autopore IV (Micromeritics, United-States) was used formercury porosimetry measurements.

Example 5—Boron Nitride Granulation

In 106 g of deionized water were dissolved 1.1 g of polyethylene glycol20M and 2.9 g of Tergitol NP10. The resulting solution is designated asthe granulation solution.

290 g of boron nitride powder (PUHP30005, Saint-Gobain) were introducedin a 1 liter high shear granulator.

The impeller rotation speed was set at 30 m/s. During the first 60seconds, the granulation solution was added continuously to the powder.During the next 180 seconds, the boron nitride was granulated. Then, thegranulation was stopped and the BN granules were dried during 24 h atroom temperature before a firing at 2050° C.

The final granules have a mean diameter of 250 μm, a sphericity of 0.97,and a porosity of 54%.

Example 6—Boron Nitride Granulation with Dusting

In 106 g of deionized water were dissolved 1.1 g of polyethylene glycol20M and 2.9 g of Tergitol NP10. The resulting solution is designated asthe granulation solution.

290 g of boron nitride powder (PUHP30005, Saint-Gobain) were introducedin a 1 liter high shear granulator. 58 g of the same powder was preparedin a beaker and designated as dusting powder.

First step: The impeller rotation speed was 30 m/s. During the first 60seconds, the granulation solution was added continuously to the powder.During the next 180 seconds, the boron nitride was granulated.

Second step: At 240 seconds, the impeller rotation speed was turned downto 3 m/s. During the next 160 seconds, the dusting powder, boron nitridepowder (PUHP30005, Saint-Gobain), was added continuously.

Then, the granulation was halted and the BN granules were dried during24 h at room temperature (see FIG. 1) before firing at 2050° C. Thefinal granules have a mean diameter of 224 μm, a sphericity of 0.99, anda porosity of 52%.

Example 7—Hybrid Boron Nitride Granulation

In 78.19 g of deionized water were dissolved 0.81 g of polyethyleneglycol 20M and 2.3 g of Tergitol NP10. The resulting solution isdesignated as the granulation solution.

232 g of boron nitride powder (PUHP30005, Saint-Gobain) and 58 g of A16SG alumina powder (Almatis, Ludwigshafen, Germany) were introduced ina 1 liter high shear granulator.

The impeller rotation speed was set at 30 m/s. During the first 20seconds, the powders were mixed together without liquid introduction.Then during the next 60 seconds, the granulation solution was addedcontinuously to the powder. During the next 180 seconds, the boronnitride and alumina were granulated to form hybrid granules. Then, thegranulation was halted and the BN/Al₂O₃ granules were dried during 24 hat room temperature before a firing at 1800° C.

The final granules have a mean diameter of 275 μm, a sphericity of 0.95,and a porosity of 40%.

Example 8—Hybrid Boron Nitride Granulation

In 78.19 g of deionized water were dissolved 0.81 g of polyethyleneglycol 20M and 2.3 g of Tergitol NP10. The resulting solution isdesignated as the granulation solution.

232 g of boron nitride powder (PUHP30005, Saint-Gobain) and 58 g ofElkem Microsilica Grade 920 silica powder were introduced in a 1 literhigh shear granulator.

The impeller rotation speed was set at 30 m/s. During the first 20seconds, the powders were mixed together without liquid introduction.Then during the next 60 seconds, the granulation solution was addedcontinuously to the powder. During the next 180 seconds, the boronnitride and silica were granulated to form hybrid granules. Then, thegranulation was turned off and the BN/SiO₂ granules were dried during 24h at room temperature before a firing at 1600° C.

The final granules have a mean diameter of 163 μm, a sphericity of 0.98,and a porosity of 49%.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions, and the like canbe made without departing from the spirit of the invention and these aretherefore considered to be within the scope of the invention as definedin the claims which follow.

What is claimed:
 1. A powder, consisting essentially of boronnitride-based aggregates, wherein the aggregates comprise: boron nitridein a hexagonal structure; and a mean circularity of at least 0.90,wherein the powder has a calcium content of less than 400 ppm by weight.2. The powder of claim 1, wherein the boron nitride-based aggregateshave a median size less than 500 microns.
 3. The powder of claim 1,wherein the boron nitride-based aggregates have an oxygen content ofless than or equal to 2% by weight.
 4. The powder of claim 1, whereinthe boron nitride-based aggregates comprise: a median pore size lessthan or equal to 0.3 microns, an open porosity less than or equal to55%; or any combination thereof.
 5. The powder of claim 1, wherein thepowder comprises in percentages by weight: between 40 and 45% inclusiveof boron; between 53 and 57% inclusive of nitrogen; and less than 5%, intotal, of other elements.
 6. The powder of claim 1, wherein the powdercomprises a structural composition including more than 90% inclusive ofboron nitride, in percentages by weight and on the basis of the totalityof the crystallized phases present in the powder.
 7. The powder of claim1, wherein the powder comprises a content by weight of boron is greaterthan or equal to 41% and a content by weight of boron is less than orequal to 44% and impurities of less than 4%.
 8. The powder of claim 7,wherein the powder comprises a content by weight of nitrogen greaterthan or equal to 54% and less than or equal to 56%.
 9. A composition,comprising a polymer and the powder of claim
 1. 10. A powder, consistingessentially of boron nitride-based aggregates, wherein the aggregatescomprise: boron nitride in a hexagonal structure; and a mean circularityof at least 0.90, wherein the boron nitride-based aggregate comprises anopen porosity less than 49%.
 11. The powder of claim 10, wherein theboron nitride-based aggregates comprise the open porosity of less than45%.
 12. The powder of claim 10, wherein the boron nitride-basedaggregates comprise a median pore size less than or equal to 0.25microns.
 13. The powder of claim 10, wherein the boron nitride-basedaggregates comprise a mean circularity greater than or equal to 0.93.14. The powder of claim 10, wherein the boron nitride-based aggregatescomprise a median size greater than 30 microns and less than 500microns.
 15. The powder of claim 10, wherein the boron nitride-basedaggregates comprise: a maximum size less than 1 mm; a percentile D10greater than 5 microns; a ratio (D90-D10)/D50 less than 10; or anycombination thereof.
 16. The powder of claim 10, wherein the boronnitride-based aggregates comprise a maximum size less than 750 microns.17. The powder of claim 10, wherein the boron nitride-based aggregatescomprise randomly oriented boron nitride platelets.
 18. The powder ofclaim 10, comprising an oxygen content less than 5000 ppm by weight. 19.The powder of claim 10, comprising a content by weight of a sinteringadditive greater than or equal to 0.5% and less than 4%.
 20. The powderof claim 10, comprising in percentages by weight and on the basis of thecrystallized boron nitride phases more than 60% of boron nitride presentin a form of a hexagonal.